United States Office of EPA 520,11-89-016Environmental Protection Radiation Programs June 1989Agency Washington. DC 20460

Radiation

�WI KYEPA Evaluation of

Skin and IngestionExposure Pathways

This report was preciared as an account of work sponsored by an agency of theUnited StatesGovernmen' Neither theUnitedStatesGovernment norany agencythereof, nor any of their employees. makes any warranty. express or implied, orassumes any legal liability or responsibility for the accuracy, completeness. orusefulness cit any normation, acioaratus. product, or process disclosed. orrepresents that its usewould not infringe privately owned rights. Reference hereinto any secific commercial product. process. or service by trade name, trademark.manufacturer. or otherwise. does not necessarily constitute or imply itsendorsement. recommendation, or favoring by the United States Government orany aenCv thereof. The views and opinions of authors expressed herein do notnecessarily state or reflect tose of the United S tates Government or any agencythereof.

Evaluation ofSkin and IngestionExposure Pathways

Rosanne Aaberg?acific. Northwest Laboratory

Richland, Washington

Joe E. LogsdonProject Officer

Office of Radiation ProgramsU.S. Environmental Protection Agency

Washington, DC 20460

1989

FOREWORD

After a nuclear accident when there has been a release of radionuclidesinto the atmosphere with consequential deposition on the ground, decisions arenecessary on whether protective action guides should be implemented. In orderto do this, several pathways for radiation exposure must be evaluated todetermine the projected dose to individuals.

The objective of this study, conducted by Pacific Northwest Laboratoriesfor the U.S. Environmental Protection Agency, is to provide backgroundinformation on exposure pathways for use in the development of ProtectiveAction Guides. The relative importance of three exposure pathways that areusually considered to be unimportant compared to other pathways expected tocontrol relocation decisions following a nuclear power plant accident isevaluated. The three pathways are the skin dose from contact withradionuclides transferred from the ground, the skin dose from radionuclides onthe ground surface, and ingestion of radionuclides transferred directly to themouth from the hands or other contaminated surfaces. Ingestion ofcontaminated food is not included in this evaluation, except for situationswhere the food is contaminated as a result of actions by the person whoconsumes the food (e.g., transfer of contamination from hands to food).

Estimates of skin and ingestion doses are based on a source term with aradionuclide mix predicted for an SST2-type nuclear accident in an area wherethe first year reference whole-body dose equivalent from whole body externalexposure to gamma radiation plus the committed effective dose equivalent frominhalation of resuspended radionuclides is rem.

Appendixes have been included to allow the reader to examine dose factorcalculations, source-term data, and quantification of contact and ingestionparameters in more detail.

Richard J. u nd, DirectorWashington, DC Office of R tion Programs

CONTENTS

Page

1. Introduction . . . . . . . . . . . . . . . . . . . . . . 1

2. Summary . . . . . . . . . . . . . . . . . . . . . . . . 22.1 Contact Pathway Dose . . . . . . . . . . . . . . . 42.2 Surface Pathway . . . . . . . . . . . . . . . . . 42.3 Ingestion Pathway . . . . . . . . . . . . . . . . 5

3. Methods and Assumptions . . . . . . . . . . . . . . . . 53.1 SST2 Source Term . . . . . . . . . . . . . . . . . 5

3.1.1 Source Term Averaging . . . . . . . . . . . 63.1.2 Weathering . . . . . . . . . . . . . . . . 7

3.2 Dose Factors . . . . . . . . . . . . . . . . . . . 83.2.1 Skin Contact . . . . . . . . . . . . . . . 83.2.2 External . . . . . . . . . . . . . . . . . 93.2.3 Ingestion . . . . . . . . . . . . . . . . . 9

3.3 Exposure Assumptions . . . . . . . . . . . . . . . 93.3.1 Skin Contact . . . . . . . . . . . . . . . 103.3.2 External . . . . . . . . . . . . . . . . . 113.3.3 Ingestion . . . . . . . . . . . . . . . . . 12

4. Results . . . . . . . . . . . . . . . . . . . . . . . . 13

5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 14

6. References . . . . . . . . . . . . . . . . . . . . . . . 17

Appendixes

A. Dose Factor Calculations and Data . . . . . . . . . . . 19

B. Source-Term Data . . . . . . . . . . . . . . . . . . . . 25

C. Quantification of Contact and Ingestion Parameters 29

V

EVALUATION OF SKIN AND INGESTION EXPOSURE PATHWAYS

1. INTRODUCTION

Proposed Protective Action Guides (PAGs) for relocation are based on

the whole-body dose equivalent from 1-yr external exposure to gamma

radiation lus the committed effective dose equivalent from inhalation of

resuspended airborne radioactive materials. The guides also include dose

equivalent limits to the skin from 1-yr beta exposure. This study was

conducted by Pacific Northwest Laboratory (PNL) for the U.S. Environmental

Protection Agency (EPA) to determine the relative importance of three

additional exposure pathways that are usually considered to be unimportant

compared to other pathways expected to control relocation decisions

following a nuclear power plant accident. The dose equivalent from each

of the following exposure pathways was evaluated and compared with the

total dose from the external gamma and inhalation exposure pathways:

Contact - from beta-emitting radionuclides transferred from the

ground to the skin;

Surface external) - from the beta emitters deposited on the ground;

and

Ingestion - from the ingestion of radionuclides transferred directly

to the mouth from the hands or other contaminated surfaces.

Ingestion of contaminated food is not included in this study.

The dose calculations are based on the assumption of no protective actions

beyond normal bathing, clothing, and partial occupancy in low or

noncontaminated areas.

2. SUMMARY

The estimated dose equivalent to the skin from beta radiation and the

committed effective dose equivalent from ingestion for the maximally exposed

individual are given in Table 1. The estimated dose to the skin includes

contributions from both contact and external sources; the committed effective

dose equivalent from ingestion results from transfer of hand contamination to

the mouth.

The doses in Table are from the first-year exposure following an SST2-

type reactor accident (Aldrich et al. 1983) of an individual who resides where

the projected effective dose from external gamma radiation plus the committed1effective dose from inhalation of resuspended materials is I rem . That is,

the doses are based on concentrations of radionuclides deposited per unit area

on the ground that would yield a reference whole-body dose rate of rem from

first-year exposure, as calculated with the CRAC2 computer code (Ritchie et

al. 1984). A first-year average source term, calculated with the radioactive

decay and the weathering functions from WASH-1400, is used in CRAC2. For this

study, the contamination layer is assumed to be incorporated in a 1-mm-thick

surface layer of soil, or a 0.1-mm-thick layer of dust associated with a paved

surface.

Contact and surface (external) beta doses are computed as rem to

radiosensitive tissues at a depth of 70 pm (ICRP 1977). This depth was

selected to correspond to assumptions used in calculation of doses supporting

the selection of PAGs for skin.

The integrated dose considering radioactive decay and weatheringaccording to the WASH-1400 model (using the CRAC2 computer code)normalized to rem for the first-year exposure is herein referred toas the "reference whole-body dose." Gamma exposure dominates this dose.

2

TABLE 1. Dose for the First-Year Exposure to Maximally Exposed(a)Individuals Residing in the 1-REM/Year "ReferenceWhole-Body Dose" Zone Following an SST2-Type ReactorAccident

First-Year'Dose (rem)(b)Individual Exposure Pathway Soil Pavement(c)

AdultContact (skin) (d) 0.7 6.7Surface (skin) (e) 1.7 1.7

TOTAL SKIN 2.4 8.4

Ingestion (e) 0.01 0.1

ChildContact (skin) 0.7 6.7Surface (skin) 7.4 7.4

TOTAL SKIN 6_.1 14

Ingestion 0.05 0.5

(a) See Table 2 for, average individuals.

(b) Skin dose is expressed as dose equivalent from the first-yearexposure and ingestion dose is expressed as the 50-yr committedeffective dose equivalent from the first-year exposure.

(c) Surface Mxing: mm for soil 160 g/cm2), 0.1 mm for pavement(16 g/CM2 .

(d) Contact assumed: contamination residing on the skin 4380 hlyrequivalent to 1.8 mg/cm2, or 1.1 percent of ground concentrationfor soil, 11 percent of ground concentration for pavement.

(e) External (surface) dose based on 4380 h/yr at elevations of mfor adults and 30 cmfor children.

(f) Based on ingestion f 00 mg/d for the adults; 500 mg/d forchildren.

3

2.1 CONTACT PATHWAY DOSE

Skin contact doses in Table were calculated for maximally exposed

individuals who spend much time outdoors, work in soil, and bathe

infrequently. The critical group includes children playing in a yard or

playground. Contact doses depend on the deposition of contaminated

materials on the skin and duration of exposure to the skin, which in turn

depends on bathing practices. The maximally exposed individual is

assumed to have a dirt layer of 1.8 mg/cm 2on the skin (1 to 11 percent

of the ground surface concentration). This is based on a 50-um layer of

dust or dirt on the skin (Hawley 1985). Dose factors for contact doses

are based on a skin depth of 70 gm.

For a given concentration of contaminants on the skin, the dose that

is calculated is the same regardless of whether a large or small area is

exposed. Although the skin is considered an organ, the dose to the skin

is not averaged over the entire skin. The dose of interest is to the

particular area of skin that receives irradiation from direct contact

with contamination. The dose is calculated to unclothed skin only

because this reflects the maximum dose rate. Contamination is assumed to

be in contact with the skin for 4380 h (from cycles of contamination,

bathing, and recontamination) for the first year after deposition of

contaminants.

2.2 SURFACE PATHWAY

Doses resulting from the exposure of ground-deposited beta emitters

are dependent on the amount of time the individual spends outdoors in a

contaminated area. The maximally exposed individual is assumed to spend

8 h/d outdoors in the contaminated area. The first-year dose to the skin

from external exposure to beta emitters 1 m above the contaminated ground

is conservatively calculated to be 07 rem/yr, based on CRAC2 weathering,

which is very conservative for beta exposure. The dose factors for

external exposure are-based on a skin depth of 70 m, as are contact dose

factors.

4

Dose factors at different distances are highly dependent on the beta

spectra. A reference height of 30 cm above contaminated ground is used

to estimate the potential dose to children. For the mix of radionuclides

in the SST2 source term, the dose a 30 cm would be approximately four

times greater than that at 1 m. The first-year beta dose at 30 cm

corresponds to about 2 to 3 rem to the skin with no credit for shielding

provided by clothing.

2.3 INGESTION PATHWAY

Ingestion doses evaluated here are only those resulting from

exposure to contaminated surfaces and from poor hand-washing practices.

For the ingestion pathway, the maximally exposed individual is a person

with contamination on the hands who does not wash his/ her hands before

eating. The critical group includes children playing outdoors and then

not washing their hands before eating, or small children who put their

hands or other objects in their mouths while playing in a contaminated

area. The ingestion pathway may potentially be a significant route. The

effective dose equivalent from the first-year intake by the maximally

exposed child may equal the I rem/yr of the reference whole-body dose.

Maximally exposed adults include individuals who engage in frequent

hand-to-mouth activity (smokers) and also engage in outdoor activities,

such as construction workers or gardeners.

3. METHODS AND ASSUMPTIONS

3.1 SST2 SOURCE TERM

The source term used for this task is based on the initial mix of

radionuclides predicted for an SST2-type reactor accident in sufficient

quantity to produce a 1-rem first-year reference whole-body dose.

In-growth of daughters as well as physical decay and weathering

corrections are applied to the source term to yield first-year average

radionuclide concentrations on surfaces. Doses from individual

radionuclides are calculated and summed.

5

3.1.1 Source-Term Averaging

The first-year average ground concentration (Ci) for each radionuclide

is determined as follows:

(I XitCi = Coi e

Xi twhere co = initial ground concentration (Ci/M2 ) of each radionuclide

i = individual radionuclide

x = effective decay constant (yr

(In 2/physical half-life) n 2/weathering half-time)

t = time period (1 yr).

The contribution from daughter nuclides using simple decay chains is

calculated as follows-

X-t -Xitd = Ci + (1 e- J e

t t

where C average ground concentration of daughter radionuclide

i, including contribution from parent radionuclide j

C initial concentration for the parent radionuclide

X decay constant of radionuclide i.

Source-term and chain-decay data used in the calculations are given in

Appendix B, Source-Term Data.

The value of C 0 is equal to the surface concentration of nuclides

comprising the SST2 source term whose sum produces a 1-rem/yr reference

whole-body dose from the gamma exposure pathway, plus the inhalation

exposure pathway, as calculated with the CRAC2 computer code (Ritchie et al.

1984). The reference dose is mostly from external exposure to

gamma-emitting radionuclides that are on the ground; inhalation contributes

6

only a small fraction of the total. The relative quantity of each

isotopeused to calculate skin and ingestion doses is directly proportional

to the SST2 particulate material source term.

3.1.2 weathering

The skin and ingestion doses are evaluated using the weathering model

from WASH-1400 (USNRC 1975) that is used in the CRAC2 computer code (Ritchie

et al. 1984). The basis for this weathering model is gamma dose rates from

soils with 137 Cs contamination distributed on the surface. Weathering of

paved areas such as sidewalks, driveways, and streets may be described by

the WASH-1400 model; the model has been confirmed to some degree by field

studies (warming 1982, 1984).

This weathering model introduces extra conservatism into the

calculation of beta dose rates. ispersion of gamma emitters into a thin

layer of soil would have a negligible effect on external exposure from gamma

radiation, but it would serve as a barrier to beta emitters. For example,

the range of a beta particle with a maximum energy of 1 MeV is about 3 mm in

�Oil, indicating that most of the beta energy will be attenuated by a soil

cover of a few millimeters. The dose rate from gamma radiation, however, is

attenuated to 10 percent of its original value only with a thickness of

about 30 cm of soil, or 100 times the range of a MeV beta particle.

Assumptions concerning mixing of the surface layer, which are related

to weathering, lead to important consequences in the calculation of contact

doses. There is an effective 1600-fold concentration difference between

contamination existing in surface dust layer of 10 g1m 2 and contamination

dispersed in the top cm of soil. Contamination that is not at the surface

would still be available for direct skin contact during gardening or field

work, but at a oncentration that has been reduced by dilution with soil.

For this study, the contamination layer is assumed to be incorporated in a

I-mm-thick 1600 /m 2) surface layer of sil, or a 0.1-mm-thick2(160 g/m layer of dust associated with a paved surface. The conversion

from mass to area is discussed in Appendix C.

7

Since leaching with rainwater could make a large difference in

surface concentrations of contaminants, the WASH-1400 weathering model

applied to beta emitters may best describe the surface conditions in a

relatively dry, unirrigated area. In the studies by Warming,

precipitation was sufficient to produce runoff only 3 percent of the time

in which precipitation occurred. The dose reduction due to weathering

could be greater in areas with heavier precipitation.

Experimental evidence shows that hosing of paved surfaces such as

sidewalks, driveways, and streets decreases the (gamma) dose rate by only

15 to 25 percent (Warming 1982, 1984). In these experiments, old asphalt

surfaces showed no significant weathering. 'It may be extremely

conservative to apply dose rates based on gamma emitters to contact beta

dose or ingestion because experiments show a marked decrease with time for

the ability to decontaminate (transfer contamination from) these surfaces.

3.2 DOSE FACTORS

Dose factors for skin contact found in the literature included

photons and electrons, and were not in the form required for this report.

Therefore, contact dose factors were developed for this task. Dose

factors for external exposure from beta radiation at 1 m were taken from

the literature (Kocher 1981b). VARSKIN, a PNL-developed computer code

(Traub et al. 1987), was used to estimate the ratio of dose factors at

different distances above the ground and to provide factors for

calculating doses at 30 cm above contaminated ground (see Appendix A for

dose-factor data). Skin dose is expressed as dose equivalent, and

ingestion dose is expressed as committed effective dose equivalent. The

exposure period for both is one year. Dose factors for ingestion are

based on ICRP 26 and ICRP 30 models (ICRP 1977, 1979).

3.2.1 Skin Contact

Radioactive material deposited directly on body surfaces is

considered to be on an infinite, thin plane. This is valid for very thin,

curved surface with a radius of curvature greater than the maximum

8

beta-particle range. Skin contact dose factors for a skin thickness of

70 um were computed using the Loevinger, Japha, and Brownell 1956)

solution to the thin-plane problem. The dose equation was solved for each

beta and electron energy level or group in the spectrum for each

radionuclide of interest. Healy 1971) used the same formula to calculate

allowable contamination levels, but used only the principal beta energies

and included photon dose in the results. Considering these differences

our calculated dose factors compared reasonably well with those by Healy.

Beta and electron spectra used in computations are those given by

Kocher (1981a). A low-energy cutoff of 100 keV (maximum energy) for beta

particles and 60 keV for electrons was assumed. Mono-energetic electrons

were treated as betas for this analysis. Add itional information,

including equations and beta spectrum data used in SKINDOSE (developed for

this study) to calculate the dose factors, is given in Appendix A.

3.2.2 External

External dose equivalent to skin from nearby beta radiation sources

was calculated for radiosensitive tissue at a single depth of 70 um (ICRP

1977). Dose factors are from Kocher (1981b), and estimates of dose

factors based on distance of 1 ft above a surface are given in Appendix A,

Table A.2.

3.2.3 Ingestion

Committed effective dose equivalents for ingestion are based on the

concepts of ICRP 26 and ICRP 30 (USEPA 1988). The dose factors for

ingestion are presented in Appendix A, Table A.1. Where two values of

dose factors (committed effective dose equivalent) based on solubility

were presented, the larger value was used in this analysis.

3.3 EXPOSURE ASSUMPTIONS

Dispersion of contaminants in soil and transfer of contaminants to

skin and to the mouth are key parts of dose estimation.

9

3.3.1 Skin Contact

The dose resulting from contamination on the skin is a function of

the concentration and duration of contaminants on skin. The concentration

of contamination on the skin is estimated based on concentration of

contaminants present on the ground over the first year after contamination

and the amount of dust or dirt residing on the skin. For this study, the

contamination layer is assumed to be incorporated in a surface layer of

soil mm thick 1600 g/m2) or a layer of dst 0.1 mm thick 160 g/m 2)

associated with a paved surface. Justification for the mixing layers is

provided in Appendix C.

The maximally exposed individual is assumed to have a dirt layer of21.8 mg/cm on the skin (1 to 11 percent of the thickness of the

contaminant ground layer). The average individual is assumed to have

1.0 mg of contaminated dust per cm2 of skin 06 to 6 percent of the

thickness of the contaminated layer). These estimates are based on

interpretations of experiments involving contamination on skin surfaces

(Hawley 1985, Shaum 1984). Conversion from mass to area is discussed in

Appendix C.

The transfer of contaminants to the skin is assumed to be

proportional to the concentration of contaminants on the ground. Because

the contamination on the ground is assumed to be mixed in a 160 to

1600-g/m2 dust layer, a dirt loading on the skin of 16 to 160 mg/cm 2

would be required to be equivalent to 100 percent of the concentration

present on the ground.

Although weathering is considered in the radionuclide source term,

the weathering model is conservative for beta emitters. Although dilution

of contaminants on the ground surface (mixing contamination into a thin

layer of soil) would have negligible effects on external exposure from

gamma emittersl it could decrease the dose from beta emitters by a

significant amount.

Despite the fact that most contamination would be on the hands and

arms, other areas of the body may also be affected (e.g., children with

10

legs uncovered sitting on a contaminated surface). Skin contact also

applies to surfaces of the feet. Although contact may be considerable

for someone with bare feet, the skin of the feet is to 10 times thicker

than skin on other areas of the body (Whitton 1973). Thus, beta doses to

sensitive tissues of the feet will be considerably less than that

estimated using a 70-um depth.

The dose equivalent for a maximally exposed individual is calculated

using the assumption that the affected skin area is contaminated for

4380 h (half the number of hours in a year) in the year following the

contamination event. The average individual is assumed to have

contamination on a portion of the skin area for 800 hlyr. These time

periods, which are assigned arbitrarily, include many recontamination

events in the course of a year.

3.3.2 External

External exposure is based on the number of hours per year the

individual is exposed to the contaminated surface (outdoors). Dose

factors for external exposure of the skin (beta radiation only) are based

on a height of 1 m above the contaminated surface2 (Kocher 1981b).

Dose factors are greater closer to the ground. Factors for 30 cm above a

contaminated surface were estimated using VARSKIN (Traub et al. 1987). A

ratio of dose factors (which depends on the energy of the particle) was

calculated by representing 30 cm and 1 m of air, plus tissue thickness of

0.007 cm with the equivalent thickness of unit density. Dose factors

from Kocher (1981b) were multiplied by this ratio to yield an estimate of

dose factors at 30 cm. To be conservative, no credit was taken for the

protection afforded by clothing.

The maximally exposed individual is assumed to be exposed to

external beta radiation for 4380 h/yr. This is equivalent to an

occupational exposure for someone who works outdoors in the

2 One meter is the standard height given in dose factor tables. Dosefactors from beta radiation are functions of height above the groundand beta energy. For a height of 1 m in air, the minimum electronenergy giving a non-zero dose-rate factor is about 320 keV; at groundlevel (0.01 m) the cutoff is about 75 keV (Kocher 1981b).

contaminated area 12 h/d, year-round. An average individual is assumed to

spend 2080 h/yr outdoors, or 40 h/wk. The exposure times for the

maximally exposed and average child are assumed to be equivalent to those

for adults.

3.3.3 Ingestion

Ingestion of radioactive surface contamination can occur when

radiocontaminants are transferred to the mouth via hands or foodstuffs.

Recent developments in estimating the ingestion of soil and dust (LaGoy

1987) are used to estimate doses to individuals. A discussion of

assumptions involved in ingestion calculations is given in Appendix C.

Because the source term for this study is given in terms of curies

per unit area, and previous studies involving radiocontaminants have used

concentrations per unit mass, ingestion of contaminants and contact with

contaminants are expressed in terms of surface area as well as unit mass.

For assessment of contact and ingestion doses by ass rather than

effective surface area, a conversion factor of 160 to 1600 gm 2 is used

to describe the extent of the contaminated layer. Relationships between

mass and surface area contamination for this assumption are discussed in

detail in Appendix C.

Individuals are assumed to ingest radiocontaminants proportional to

that found on a given'surface area. The amounts are based on ingestion of

contaminated soil and are compared with results from methods that were

developed for occupational exposure to radiocontaminants. Data from LaGoy

(1987) suggest ingestion rates of 100 mg/d for maximally exposed adults

and 25 mg/d for the average adult who does not participate in much outdoor

activity and does not smoke. Ingestion rates for children are taken as

500 mg for the maximally exposed child and 100 mg/d for the average

child. Since the units of the SST2 source term are in Ci/unit area, the

ingestion quantity must be converted from grams per day. This conversion

makes the source term units compatible with the units of ingestion. These

ingestion rates are equivalent to the total contaminated dust from 1. to

6.25 cm2/d (based on pavement dust) for adults and 625 to 31 cm 2/d

for children (see Appendix C).

12

The rate at which contamination is picked up from surfaces and

ingested, given by Dunster 1962) and Gibson and Wrixon 1979) is

10 cm2/d for occupational exposure (8 h/d). For exposure to the

public, this value is multiplied by 16/8 (to correct to 16 h/d of

exposure) to yield 20 cm 2M. These ingestion rates correspond

reasonably well with mass ingestion rates from LaGoy 1987) based on a

thin dust layer on a paved surface.

Children old enough to play outdoors, but young enough not to have

acquired good personal hygiene practices, are the critical group because

ingestion of contaminated soil is estimated to be higher than that for

the maximally exposed (adult) individual. According to LaGoy 1987), the

average child of to 6 yrs of age may ingest 100 mg of dust or soil per

day, and the maximally exposed child of that age (excluding those with

habitual pica) may ingest 500 mg of dust per day. Age-specific dose

factors are not used in this analysis and are beyond the scope of this

report.

4. RESULTS

Dose equivalents, based on residency in an area within the 1-rem

first-year reference whole-body dose zone for the maximally exposed and

average individuals, are summarized in Table 2 The radionuclide source

term for exposures is a fi rst-year average concentration based on the

WASH-1400 weathering model. The two columns in Table 2 correspond to

exposures based on soil (contamination mixed with the top mm of soil)

and pavement (contamination mixed with the top 0.1 mm of surface dust).

Based on the assumed conditions, the ingestion of surface

contamination is a potentially significant exposure pathway. For the

maximally exposed individual, ingestion may account for up to percent

of the reference whole-body dose for an adult or 50 percent for a child.

In order of importance, the major contributors t effective dose

equivalent from ingestion during the first year include 137 Cs

(33 percent), 134 Cs 31 percent), 132 Te -132 1 6 percent), 131I

(8 percent), and 144 Ce 7 percent). For the SST2 source term, these

isotopes account for about 85 percent of the ingestion dose equivalent.

13

Contact doses are for exposure to the skin and are not directly

comparable to the reference whole-body dose. The isotopes 132 Te- 132I

(28 percent), 137 Cs (15 percent), 144 Ce 14 percent), and 134 Cs

(7 percent) account for the major part of the dose to skin by direct

contact for the first year after SST2 contamination. Doses to the average

individual (based on 1.0 mg/cm2 on skin for 800 h) are about 10 percent

of those for the maximally exposed individual. The average individual

would also have a smaller portion of contaminated skin area.

Exposure to beta emitters at 1 m for adults or 30 cm for children

above a contaminated surface accounts for a small increment to the total

dose. The surface (external) beta dose is equal to about a tenth of the

dose from direct contact for the maximally exposed adult but about twice

the contact amount for the average child. Estimates of external exposure

to beta radiation depend only on the number of hours spent outdoors and

not on personal cleanliness or other factors. The estimates of external

beta dose are conservative; actually clothing would provide some

protection from beta radiation. To be conservative, no credit is taken

for shielding provided by clothing. The dominant contributor to external132 132

beta dose at 1 m is I (daughter of Te), which accounts for about

55 percent of the dose. Other contributors to external dose are 91Y

(17 percent) and 129M Te (9 percent). The dose estimated at 30 cm is

dominated by 132 1 39 percent), 134 Cs 17 percent), 91 Y(10 percent),

and 127 Te 9 percent). A larger proportion of the dose is from

radionuclides with softer betas.

5. DISCUSSION

Table 2 shows that dose equivalent to skin from beta emitters and

committed effective dose equivalent from ingestion via contaminated hands

may be significant compared with inhalation dose plus external dose from

gamma radiation.

The contact dose is closely related to weathering of the surface

layer of contaminants. Assumptions related to weathering have important

consequences in the calculation of contact doses. An effective 1600-fold

14

TABLE 2 Dose for the Maximum and Average Individual Residing in the1 REM/Year "Reference whole-Body Dose" Zone Following anSST2-Type Reactor Accident

First-Year Dose (rem)(a)Individual Exposure Pathway Soil Pavement(b)

Maximum Contact (skin) (c) 0.7 6.7Adult Surface (skin) (d) 1.7 1.7

TOTAL SKIN T.7 8.4

Ingestion (e) 0.01 0.1

Average Contact (skin) 0.06 0.6Adult Surface (skin) (d) 0.7 0.7

TOTAL SKIN 0.8 T_.3

Ingestion (e) 0.002 0.02

Maximum Contact (skin)(c) 0.7 6.7Child Surface (at 30 cm)(d) 7.4 7.4

TOTAL SKIN -8.1 14

Ingestion(e) 0.05 0.5

Average Contact (skin)(c) 0.06 0.6Child Surface (at 30 cm)(d) 3.2 3.2

TOTAL SKIN 3.3 3.8

Ingestion (e) 0.01 0.1

(a) Skin dose is expressed as dose equivalent from the first-yearexposure and ingestion dose is expressed as the 50-yr committedeffective dose equivalent from the first-year exposure.

(b) Surface mixing: mm for soil 160 mg/cm2), 0.1 mm for pavement(16 mg/W2).

(c) Contact assumed: contamination residing on the skin 4380 h/yrequivalent to 1.8 mg/cm2, for maximally exposed, 800 h/yr and1.0 Mg/Cm2 for average individuals.

(d) External (surface) dose based on 4380 h/yr for maximally exposed,2080 for average individuals. 1 m for adult, and 30 cm for child.

(e) Based on ingestion of mg/d for the maximally exposed adult,25 mg/d for average adult; 500 mg/d for the maximally eposed child,100 mg/d for average child.

15

concentration difference between contamination in the surface dust layer

of 10 g/M2 and contamination dispersed in the top cm of soil exist).

For this study, the contamination layer is assumed to be incorporated in

a 100 g/m2 surface layer of dust and soil. (Conversion from mass to

area is discussed in Appendix C).

A depth of 70 um is used for the calculation of skin dose (for both

contact and external components) to correspond to assumptions used in

dose calculations supporting the selection of PAGs for skin. A value of

40 um has been suggested as the appropriate depth of the radiosensitive

layer of skin (Whitton 1973). The use of dose factors calculated at the

40-um depth would increase the resulting dose by about 40 percent.

Although the skin of the hands is likely to come in contact with surface

contamination, the skin thickness is greater, reducing the potential

damage to the sensitive layer.

Absorption of contaminants through the skin is another potential

pathway for internal exposure. A study of radioiodine determined an

absorption rate for iodine through skin of 0.008 percent h cm 2

(Harrison 1963). For the maximally exposed individual, assuming

3000 cm2 of skin is contaminated for 3000 h, this would correspond to

about 35 percent of the intake of iodine by ingestion of contaminants

transferred from contaminated surfaces. This includes the assumption

that the particulate iodine in the contaminants are absorbed as well as

the aqueous solution used in the experimental procedure. A matrix of

soil rather than solvent can affect the absorption by skin; experiments

using TCDD (dioxin) showed that the soil matrix reduced the amount

absorbed by 85 percent (Hawley 1985). The soil matrix may have a similar

effect on absorption of iodine.

Many variables may affect doses to residents of a contaminated

area. In some areas, the time of year an accident occurs could have a

large effect. For.example, residents are much more likely to come in

contact with dirt in spring or summer. Also, the mix of radionuclides

will change with time as the short-lived radionuclides decay.

16

Assumptions about the transfer from surfaces to skin and the

residence time on skin are somewhat arbitrary and might be modified if

additional data become available in the future.

REFERENCES

ALDRICH, D. C., J. L. Sprung, D. J. Alpert, K. Diegert, R. M. Ostmeyer,L. T. Ritchie, and D. R. Strip. Technical Guidance for Siting CriteriaDevelopment. NUREG/CR-2239, SAND81-1549, prepared by Sandia NationalLaboratories for the U.S. Nuclear Regulatory Commission, Washington,DC, 1983.

DUNSTER, H. J. Maximum Permissible Levels of Skin Contamination.AHSB(RP)R28, United Kingdom Atomic Energy Authority, Authority Healthand Safety Branch Report, Harwell, Didcot, Berkshire, England, 1962.

GIBSON, J. A. B., and A. D. Wrixon. "Methods for the Calculationof Derived Working Limits for Surface Contamination by Low-ToxicityRadionuclides." Health Physics 36(3):311-321, 1979.

HARRISON, J. "The Fate of Radioiodine Applied to Human Skin."Health Physics 9993-1000, 1963.

HAWLEY, J. K. "Assessment of Health Risk from Exposure toContaminated Soil." Risk Analysis 54):289-302, 1985.

HEALY, J. W. Surface Contamination: Decision Levels. LA-4558-MS,Los Alamos Scientific Laboratory, Los Alamos, New Mexico, 1971.

INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION (ICRP). Recom-mendations of the International Commission on Radiological Protection,TC`R5-blication 26, Pergamon Press, New York, 1977.

INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION (ICRP).Limits for Intakes of Radionuclides by Workers, ICRP Publication 30,Pergamon Press, New York, 1979.

KOCHER, D. C. Radioactive Decay Data Tables. DOE/TIC-11026, U.S.Department of Energy, Washington, DC, 1981a.

KOCHER, D. C. Dose-Rate Conversion Factors for External Exposureto Photons and E7ectrons. NUREG/CR-1918, prepaFe-dby -Oak RidgeNational Laboratory for the U.S. Nuclear Regulatory Commission,Washington, DC, 1981b.

LAGOY, P. K. "Estimated Soil Ingestion Rates for Use in RiskAssessment". Risk Analysis 73):355-359, 1987.

17

LOEVINGER, R., E. M. Japha, and G. L. Brownell. "DiscreteRadioisotopes Sources," Chapter 16. In Radiation Dosimetry, eds. G. H.Hine and G. L. Brownell, pp. 693-799, Academic Press, Inc., New York,1956.

RITCHIE, L. T., D. J. Alpert, R. P. Burke, J. D. Johnson, R. M. Ostmeyer,D. C. Aldrich, and R. M. Blond. CRAC2 Model Description.NUREG/CR-2552, SAND82-0342, prepared by Sandia National Laboratories,Albuquerque, New Mexico and Livermore, California for the U.S. NuclearRegulatory Commission, Washington, DC, 1984.

SCHAUM, T. Risk Analysis of TcDD Contaminated Soil. Prepared forEPA Office of Health and Environmental Assessment, Washington, DC, 1984.

TRAUB, R. J., W. D. Reece, R. 1. Scherpelz, and L. A. Sigalla.Dose Calculation for Contamination of the Skin Using the Computer CodeVARSKIN. NUREG/CR-4418, PNL-5610. prepared by Pacific NorthwestLaboratory for U.S. Nuclear Regulatory Commission, Washington, DC, 1987.

U.S. ENVIRONMENTAL PROTECTION AGENCY. Limiting Values of RadionuclideIntake and Air Concentration and Dose Conversion Factors forInhalation, Submersion, and Ingestion. EPA-52011-88-020. U.-S.Environmental Protection Agency, Office of Radiation Programs, 1988.

U.S. NUCLEAR REGULATORY COMMISSION. Reactor Safety Study: An Assessmentof Accident Risks in U.S. Commercial Nuclear Power Plants. AependixG: Calculation of Reactor Accident Consequences. WASH-1400 (NUREG75/014). U.S. Nuclear Regulatory Commission, Office of NuclearResearch, Washington, D.C. 1975.

WARMING, L. Weathering and Decontamination of RadioactivityDeposited on Asphalt Surfaces. RISO-M-2273, Riso National Laboratory,DK-4000 Roskilde, Denmark, 12.

WARMING, L. Weathering and Decontamination of RadioactivityDeposited on Concrete Surfaces. RISO-M-2473, Riso__Nat_`5_naT Laboratory,DK-4000, Roskilde, Denmark, 184.

WHITTON, J. T. "New Values for Epidermal Thickness and TheirImportance." Health Physics 24:1-8, 1973.

18

APPENDIX A

DOSE FACTOR CALCULATIONS AND DATA

A.1 COMPUTATION OF SKIN OSE FACTORS

Skin contact dose factors were calculated with the computer code

SKINDOSE, developed for this task. An equation presented by Loevinger,

Japha, and Brownell 1956) was used to estimate doses resulting from

radioactive materials deposited directly on body surfaces. The equation

is a function of the distance from a thin-plane source:

D(x) (1.07)v(E avg)aa{cE(l + in(c/vx) - e G VX/c) + e(1 Vx),

given ] =_ 0 for x > c/v,

where D(x) = dose rate (rad/h) at distance x (gm/cm 22v = beta absorption coefficient (cm /g)

E = average beta eergy (eV)

a = surface activity Vi/cm 2

a and c = functions of the maximum beta energy (E maLx

Emax (MeV) c

0.17 - 0.5 0.260 2.0

0. - 5 0.297 1.5

1.5 - 30 0.333 1.0

19

Skin depth x is assumed to be 70 um which equates to2 3

7.8 E-3 gm/cm for a skin density of 112 gm/cm . The equation is

solved for D(x) for unit activity for each radionuclide of interest. The

dose factor for a radionuclide is calculated from individual beta and

electron energy levels of its beta spectrum weighted by frequency of

occurrence. Dose factors for the components of the spectrum are summed

giving a dose factor for the radionuclide.

A depth of 70 pm is used to calculate skin dose (for both contact

and external components) to correspond to assumptions used in dose

calculations supporting the selection of PAGs for skin. The use of a

different depth would change the dose factors.

Beta energy levels and some electron energies are grouped to

simplify calculations. Low-energy cutoffs were made for electrons that

cannot penetrate through 70 pm of skin. In addition, contributions from

short-lived (one hr or less) daughter decay products are included in dose

factors for the parent radionuclides.

Dose conversion factors for each of the isotopes in the source term

are given in Table A.I. This table gives dose factors for skin contact,

external exposure (from 1 m and 30 cm; beta only), and ingestion. Dose

factors for skin contact are from SKINDOSE, for external exposure at m

are from Kocher (1981b), and for external exposure at 30 cm are

calculated using VARSKIN. The ingestion dose based on ICRP models is the

50-yr committed effective dose equivalent per unit of activity ingested

(USEPA 1988).

A.2 BETA SPECTRA

The beta spectrum of a radionuclide may have many components. Each

beta has two parameter energies, average (E ) and maximum (E ),avg maxand a frequency (intensity) associated with this mode of decay. The two

characteristic energies, Eavg and Emax' are used in calculating dose

factors from beta spectra. Conversion and Auger electrons, however, have

one characteristic energy rather than a range. To use the same

techniques for calculating dose factors, average and maximum energies are

20

TABLE A.I. Summary of Dose Conversion Factors for Skin Contact, External

Exposure to Beta and Electron Radiation and for Ingestion

DOSE FACTORSTa7

Nuclide Contact, rem/pCi External, rem/PCi/m 2 Or Ingestion4 rem/uCi

per cm 2 /h at 1 m at 30 cm ingestion

58CO 1.20E+00 ME-04 8.76E-04 3.6E-0360CO 4.30E+00 OE+00 OE+00 2.7E-0286Rb 8.30E+00 8.99E-01 2.25E+00 9AE-0389Sr 9.40E+00 7.88E-01 2.1E+00 9.3E-039OSr 7.20E+00 1.61E-02 4.03E-02 1.4E-0190Y 8 50E+00 1.24E+00 3.11E+00 1.1E-0291Y 8:20E+00 8.21E-01 2.09E+00 9.5E-0395Zr 5.10E+00 2.54E-03 6.35E-03 3.8E-0395Nb 8.40E-01 1.75E-03 4.37E-03 2.6E-0399MO 8.30E+00 4.18E-01 1.05E+00 5.OE-0399MTc 5.80E-01 5.62E-04 1.41E-03 6.2E-05

103Ru 2.60E+00 4.4E-03 1.1E-02 3.OE-03106Ru 8.50E+00 OE+00 OE+00 2.7E-02105Rh 5.60E+00 9.58E-03 2.4E-02 1.5E-03127Sb 7.80E+00 2.34E-01 1.37E+00 6.7E-03127Te 7.OOE+00 6.03E-02 4.16E+00 6.9E-04127MTe 2.20E+00 2.14E-03 5.36E-03 8.3E-03129Te 9.20E+00 6.59E-01 1.93E+00 2.OE-04129MTe 5.30E+00 2.83E-01 7.2E-01 1.1E-02131MTe 7.60E+00 5.. 77E-02 1.44E-01 ME-03132Te 3.OOE+00 OE+00 OE+00 9AE-03131I 9.40E+00 2.13E-02 1.'47E+00 5.3E-02132I 8.10E+00 5.59E-01 1.'69E+00 6.7E-04134CS 5.OOE+00 3.81E-02 9'53E--02 7.3E-02136CS 5.40E+00 6'25E-03 1.56E-02 1.IE-02137CS 6.90E+00 2.75E-02 6.86E-02 5.OE-0214OBa 7.40E+00 1.98E-01 4.96E-01 9.5E-0314OLa 8.60E+00 6.59E-01 1.94E+00 8.4E-03141Ce 6.80E+00 4.55E-03 1.14E-02 2.9E-03143Ce 8.40E+00 4.33E-01 1.08E+00 4.6E-03144Ce 1.20E+01 OE+00 OE+00 2.1E-02143Pr 7.80E+00 2.33E-01 1.47E+00 4.7E-03147Nd 7.20E+00 1.05E-01 2.64E-01 4.4E-03239Np 8.70E+00 2.52E-03 6.3E-03 3.3E-03238pu OE+00 OE+00 OE+00 3.2E+00239pu OE+00 OE+00 OE+00 3.5E+00240pu OE+00 OE+00 OE+00 3.5E+00241pu OE+00 OE+00 OE+00 6.8E-02241Am 2.10E-01 OE+00 OE+00 3.6E+00242CM OE+00 OE+00 OE+00 1.1E-01244CM OE+00 OE+00 OE+00 2.OE+00

(a) The contact dose factor is for beta emitters residing directly on theskin; the external dose factor is based on irradiation from contaminatedground. Both are based on dose at a 70-pm skin depth. The ingestiondose factor is based on ICRP dosimetry.

21

hypothesized. Beta and electron spectra used in the computations were

from Kocher (1981a). A low-energy cutoff of 100 keV (E max ) for beta

particles and 60 keV for electrons was used. Mono-energetic electrons

are treated as betas for this assessment. Some energy levels are grouped

for simplicity. An example of how data from Kocher for 127 Sb is

interpreted for determining skin dose factors is given in Table A.2.

Mono-energetic electrons were assumed to behave as if they were beta

particles, with a maximum energy (E max ) corresponding to that expected

for a beta particle with the same average energy. This assumption led to

reasonable results, based on comparisons of tabulated mean energy emitted

per unit of accumulated activity (column 4 in Table A.2) in units of

rad/h per pCi/g. Resulting dose factors from beta particles and

electrons were compared with mean energy emitted. They correlated well.

Table A.2. Example Application Using Data for 127SB From Kocher (1981a)as Input to Skindose

Radiation Energy Intensity Delta SKINDOSE InputType NO) (percent) (rad/hr per uCi/g) Average Maximum

Auger-L 3.19 3.9 0.0003 Below ThresholdAuger-K 22.7 0.53 0.0003 Below Thresholdce-K- 1 22.29 3.47 0.0022ce-L- 1 56.16 0.45 0.0005ce-K- 6 220.6 0.43 0.0020 221 660ce-K- 19 441.19 0.220 0.0021 441 1240cd-K- 30 653.4 0.212 0.0017 653 1740

Beta-1 max 258 0.110 0.0002avg 72.7 81 285

Beta-2 max 291 0.610 0.0011avg 82.9

Beta-3 max 425 0.8 0.0022avg 127.5 130 435

Beta-4 max 441 1.25 0.0035avg 132.8

Beta-5 max 504 5.22 0.0172 504 155.1avg 155.1

Beta-6 max 657 1.25 0.0056 657 211.1avg 211.1

22

Kocher (1981b) assessed doses from mono-energetic electrons by a

more rigorous method. The equation for mono-energetic sources included a

term for the specific absorption fraction. The value of this quantity is

evaluated by interpolation of tabulated values obtained using Monte Carlo

techniques. For this analysis, the more rigorous approach was not

required.

A.3 DOSE FACTORS FOR EXTERNAL EXPOSURE

Dose factors used for external exposure of skin to a depth of

70 pm from electrons (betas and electrons, as distinguished from photons)

are from Kocher 1981b (given in Sv/Bq/cm2). These factors are for 1 yr

of chronic exposure of skin to beta emitters at I m above a contaminated

surface and are given in Table A.1.

Dose factors for a distance of 30 cm above contaminated ground were

estimated using VARSKIN (Traub et al. 1987). A ratio of dose factors was

calculated by representing 30 cm and I m of air plus the tissue thickness

of 0007 cm by the equivalent thickness of unit density. Dose factors

from Kocher (1981b) were multiplied by this ratio to yield an estimate of

the dose factor 30 cm above contaminated ground.

The dose factors for the nuclides of interest averaged about four

times greater at 30 cm than for 1 m above contaminated ground. These

factors may be appropriate to assess external exposure from surface

contamination to children. However, the potential dose from exposure to

beta radiation from contaminated surfaces is less than the potential dose

from the contact pathway.

A.4 DOSE FACTORS FOR INGESTION

Dose factors for ingestion are 50-yr effective committed dose

following 1 yr of chronic uptake based on ICRP 26 and ICRP 30 models

(USEPA 1988). Ingestion dose factors for radionuclides of interest, in

units of rem/vCi ingested, are given in Table A.I. In cases where there

were dose factors for different chemical forms, the larger factor was

selected.

23

REFERENCES

KOCHER, D. C. Radioactive Decay Data Tables. DOE/TIC-11026, TechnicalInformation Center, U.S. Department of Energy, Washington, DC, 1981a.

KOCHER, D. C. Dose-Rate Conversion Factors for External Exposure toPhotons and Electrons. NUREG/CR-1918, prepared by the Oak Ridge NationalLaboratory for the U.S. Nuclear Regulatory Commission, Washington, DC, 1981b.

LOEVINGER, R., E. M. Japha, and G. L. Brownell. "Discrete Radio-isotope Sources," Chapter 16. In Radiation Dosimetry, eds. G. H. Hine andG. L. Brownell, pp. 693-799, Academic Press, Inc., New York, 1956.

TRAUB, R. J., W. D. Reece, R. 1. Scherpelz, and L. A. Sigalla. DoseCalculation for Contamination of the Skin Using the Computer Code VARSKIN.NUREG/CR-4418, PNL-5610, prepared by Pacific Northwest Laboratory for U.S.Nuclear Regulatory Commission, Washington, DC, 1987.

U.S. ENVIRONMENTAL PROTECTION AGENCY. Limiting Values of Radionuclide Intakeand Air Concentration and Dose Conversion Factors for Inhalation,Submersion, and Ingestion. EPA-52O/1-88-U2-O0-. Environmental ProtectionAgency, Office of Radiation Programs, 1988.

24

APPENDIX

SOURCE-TERM DATA

B.1 SST2 SOURCE TERM

The source term used for this report is based on the initial mix

of radionuclides predicted for an SST2-type reactor accident in

sufficient quantity to produce a 1-rem first-year reference whole-body

dose.

In-growth of daughters as well as physical decay and weathering

corrections are applied to the source term to yield first-year average

radionuclide concentrations on the ground surfaces. Decay chains and

the effect of weathering options are described in this appendix.

B.2 DECAY CHAINS

Table B.1 lists radionuclides in the source term and shows the

parent radionuclides used in simple decay chain calculations.

B.3 WEATHERING

Average ground concentrations of radionuclides listed in Table

B.2 are first-year average concentrations calculated with CRAC2

weathering (Ritchie et al. 1984), a two-step exponential weathering

model from WASH-1400 (USNRC 1975).

25

TABLE B.1. Radionuclides and Decay Chainsin SST2 Source Term

Nuclide Half-Life, days Parent

58CO 7.130E+0160CO 1.921E+0386Rb 1.865E+0189Sr 5.200E+019OSr 1.026E+0490Y 2.670E+00 9OSr91Y 5.880E+01 91Sr95Zr 6.550E+0195Nb .3.510E+01 95Zr99MO 2.751E+0099mTc 2.508E-01 99MO

103RU 3.959E+01106Ru 3.690E+02105Rh 1.479E+00 105Ru127Sb 3.800E+00127Te 3.896E-01 127Sb127mTe 1.090E+02129Te 4.861E-02 129Sb129mTe 3.340E+01131mTe 1.250E+00132Te 3.250E+001311 8.040E+00 131mTe132I 9.52IF-02 132Te134CS 7.524E+02136CS 1.300E+01137CS 1.099E+041408a 1.279E+0114OLa 1.676E+00 1408a141Ce 3.253E+01143Ce 1.375E+00144Ce 2.844E+02143Pr 1.358E+01 143Ce147Nd 1.099E+01239Np 2.350E+00242CM 1.630E+02238pu 3.251E+04 242CM239pu 8.912E+06 239Np244CM 6.611E+03240pu 2.469E+06 244CM241pu 5.333E+0324IAm 1.581E+05 241pu

26

TABLE B.2. Initial Deposition and First-Year Average

Concentration Calculated with CRAC2

(WASH-1400) Weatherin Model.

Average Concentration

Initial, Decay Only, CRAC2,Nuclide Ci/m2 Ci/m2 Ci/M2

58CO 1.4E-08 3.8E-09 3.2E-0960CO 1.OE-08 9.3E-09 7.OE-0986Rb 4.3E-08 3.2E-09 3.OE-0989Sr 6.5E-07 1.3E-07 1.2E-079OSr 3.2E-08 ME-08 2.3E-0890Y 1.4E-07 3.3E-08 2.5E-0891Y 3.3E-06 7.6E-07 6.6E-0795Zr .4.3E-06 1.1E-06 9.5E-0795Nb 4.3E-06 1.7E-06 1.4E-0699mo 2.1E-06 2.3E-08 2.2E-0899MTc 2.OE-06 2.4E-08 2.4E-08

103Ru 1.6E-06 2.6E-07 2.3E-07106Ru 3.7E-07 2.7E-07 2.OE-07105Rh 9.4E-07 6.OE-09 5.9E-09127Sb 1.7E-05 2.5E-07 2.5E-07127Te 1.7E-05 2.8E-07 2.8E-07127MTe 2.2E-06 8.5E-07 7.OE-07129Te 4.1E-05 3.1E-08 3.1E-08129MTe 7.8E-6 1.OE-06 9.5E-07131MTe 2.3E-05 1.1E-07 1.1E-07132Te 2AE-04 ME-06 ME-06131I 1.7E-05 6.6E-07 6AE-07132I 3.5E-05 ME-06 ME-06134CS 2.6E-06 2.2E-06 1.7E-06136CS 1.4E-06 7.4E-08 7.IE-08137CS 3.6E-06 3.6E-06 2.7E-0614OBa LIE-06 5.3E-08 5.1E-0814OLa 4.3E-06 8.2E-08 7.9E-08141Ce 4.OE-06 5.2E-07 4.8E-07143Ce 3.6E-06 1.9E-08 1.9E-08144Ce 2.7E-06 1.8E-06 IAE-06143Pr 3.7E-06 2.2E-07 2.1E-07147Nd 1.6E-06 6.8E-08 6.6E-08239Np 4.2E-05 3.9E-07 3.9E-07242CM 1.7E-08 8.4E-09 6.8E-09238pu 1.OE-09 1.IE-09 7.9E-10239pu 8.9E-10 9.OE-IO 6.7E-10244CM 1.2E-10 1.2E-10 8.7E-11240pu 8.6E-10 8.6E-10 6AE-10241pu 1.6E-07 1.6E-07 1.2E-07241AM 1.1E-10 2.4E-10 1.7E-10

27

The average concentration for CRAC2 weathering is calculated by thefollowing algorithm:

Ci = 0.63 8-R1i + 037 1 - eR2ioi RF__ R2i

where R1 i= 113 + x)t

R2i= 00075+ xOtl

t = years

The correction for in-growth of a daughter product from parent j is

as follows:

Ci = C + j Xi 0.63 (1 - eR1j)- (I - eRli)Xi xi R1j Rli

+ O.37 (1 - e-R2j) (1 - e-R2i)

R2j R2i

where xi and xj are the decay constants of nuclides i and j.

REFERENCES

RITCHIE. L. T., D. J. Alpert, R. P. Burke, J. D. Johnson, R. M. Ostmeyer,D. C. Aldrich, and R. M. Blond. 1984. CRAC2 Model Description.NUREG/CR-2552, SAND82-0342, prepared by Sandia National Laboratories.

U.S. NUCLEAR REGULATORY COMMISSION. Reactor Safety Study; An Assessmentof Accident Risks in U.S. Commercial Nuclear Power Plants. Appendix6: Calculation of Reactor Accident Consequences. WASH-1400 NUREG7-570-147. U.S. Nuclear Regulatory Commission, Office of NclearResearch, Washington, D.C., 1975

28

APPENDIX C

QUANTIFICATION OF CONTACT AND INGESTION PARAMETERS

C.1 SURFACE CONTAMINATION/MIXING ASSUMPTIONS

Exposure to two types of surfaces with differing contaminant

concentrations is hypothesized. The first is a hard surface (paved) on

which a very thin layer of dust is present; the second surface is soil,

which has a thicker mixing layer.

For the hard surface, the amount of dust in which contamination is2mixed is assumed to be 160 g/m . This corresponds to a dust layer

0.1 mm thick, based on a density of 16 g/cm 3, or approximately year

of dustfall in a moderately dusty area (Hawley 1985). Hawley gives

dustfall for the community of Niagara Falls as 12 mg/cm 2 per 30 days,

or 400 mg/m2/d, which is described as moderate dustfall. A dust layer

of 160 g/m2 is year of dustfall accumulating at a rate 10 percent

higher than the rate for Niagara Falls.

For comparison, the mass of a dust layer was calculated using mass

and concentration data for lead. Using lead in dust measurements

reported by Gallacher et al 1984) and load concentration data from

Duggan and Williams 1977) or Lpow et al 1975) results in an stimate

of the mass of the dust layer on paved surfaces that is mch lower than

the 160'g/m2 given above. The quantity of lead in dust per unit area

from a study in wales (Gallacher et al 1984) was given as 8.5 mg/m 2 for

paved areas adjacent to houses on roads with heavy traffic and

2.7 mg/m2 for similar areas on cul-de-sacs. Typical lead

concentrations in street dust measured in residential areas in greater

29

London range from 920 to 1840 ppm (Duggan and Williams 1977); A U.S.

study (Lepow et al 1975) reported mean lead levels in street dirt of

1200 ppm. Assuming concentration of lead in dust in the range of 50 to2

2000 pg/g, the mass of dust would be only 13 to 17 g/m This

corresponds to a thickness of 0.01 mm or less. This estimate of the dust

layer seemed unreasonably thin and was not comparable with measured

dustfall.

The choice of dust thickness is rather arbitrary due to the large

number of variables affecting dust deposition and accumulation. Some of

the variables include roughness of surface, wind frequency and velocity,

and amount and intensity of rainfall. The value of 160 g/m 2 was chosen

as the mixing layer for paved surfaces. A smaller value comparable to

the estimates based on lead deposition may be more appropriate for some

circumstances, but this assumption will be offset, to some degree, by

assumptions for the amount of dust transferred to skin from paved

surfaces (see Section 20).

For a nonpaved surface, a mixing layer of mm, or 1600 g/m 2 was

used. The mm thickness is used in an attempt to describe a surface

that is mixed by forces other than cultivation. A relatively thin layer

was used to provide conservatism (i.e., to bias the dose calculation on

the high side, if at all). Concentrations may be calculated with a

greater mixing depth, but this would provide additional dilution, thus be

less conservative.

C.2 DUST/DIRT RESIDING ON SKIN SURFACES

The amount of dust or dirt that may reside on the skin of a2

maximally exposed individual is assumed to be 1.8 mg/cm . The dirt

loading on the skin of an average individual is assumed to be2

1 mg/cm . These values may be high (conservative) for paved surfaces

where the relative roughness of the surface tends to reduce the amount of

dust transferred to the skin. The quantities are based on estimates by

Schaurti 1984) and Hawley 1985), and studies by Lepow 1975) and Roels

(1980). The range of 0.5 to 1.5 mg/cm 2 was assumed by Schaum 1984 to

represent an average value for the entire exposed area of the body.

30

Hawley 1985) estimates the dirt loading on the skin of an adult

equivalent to 1.8 mg per cm 2, which is based on a 50-um layer of dust

with density of 07 g/cm 3 (to account for the voids between dust

particles, however, an effective density of 035 g/cm 3 was used).

The Lepow 1975) study deals with ingestion of lead from dirt on the

hands of children. Preweighed self-adhesive labels were used to sample

dirt on the hands. The mean weight of hand dirt samples was 11 mg for a2 221.5-cm label, or 0.5 mg/cm . This weight was assumed to be a lower

bound for soil residing on the skin of children because the labels are not

100 percent efficient in transferring contamination. The estimate of dirt2on skin derived from the Roels et al. 1980) study is 1.5 mg/cm , which

is based on the quantity of lead on the hands of 11-year-old children

playing on a playground.

The concentration of contamination present on the surface of the skin

may be calculated from the quantity of contaminants per unit area on the

ground, the mixing depth and mass, and the mass per unit area on the skin.

This concentration may be calculated as a fraction of the contamination on

the ground, as follows:

2 2 mg/cm2 (skin) g 10000 cm 2Ci/m (skin) = Ci/m (ground) x 2 x - x 2

g/m (ground) 1000 mg M

For a surface contamination mixed in 160 g/m 2 of dust and 1.8 mg/cm 2 on

the skin of the maximally exposed individual the fraction of the

concentration of contaminants from the ground that are on the skin would be

1.8/160 x 10, or 0.11 (11 percent of the ground surface concentration). For a

1-mm mixing layer (contaminated layer 1600 g/m 2), the fraction on the skin

would be 1.1 percent of the ground oncentration. In order to have 100

percent of the contamination from the ground on the skin surface, the dirt2loading would have to be 16 mg/cm . Table C.1 gives the-ratio of

skin-to-ground contamination for the assumptions for surface mixing and

loading of dirt on the skin. The dirt loading on the skin is assumed not to

31

be a function of age, which means the individual referred to in Table C.1

is the same for an adult or child.

Table C.1 Fraction of Ground Concentration Assumed to be on the SkinSurface.

Fraction of Surface Contamination on SkinSurface Mixing Maximum Individual 2 Average Individual 2

(loading = 1.8 mg/cm (loading = mg/cm

Pavement 160 gm2 0.11 0.063

Ground 1600 g/m2 0.011 0.0063

The activity residing on skin of the maximally exposed individual is,

therefore, estimated to be 1.8 mg/cm 2 divided by 16 mg/cm2 for a dust

layer on pavement, or 11 percent of the total ground concentration. For a

soil surface, this is equivalent to 1.1 percent of the contamination mixed

in the 160 mg/cm 2surface soil layer. For the average individual, the2contamination is assumed to be 1.0 mg/cm , or about 6 percent of the

dust layer or about 06 percent of the surface soil layer.

C.3 INGESTION OF DIRT/DUST

Ingestion of contaminants can occur when surface contamination is

transferred from a surface to hands, foodstuffs, cigarettes, or other

items. Ingestion of dirt from contaminated hands has been investigated in

the research of lead ingestion by children and more recently in the

context of exposure to other hazardous contaminants. Soil ingestion

studies have been used to estimate the amount of soil on skin surfaces and

age-dependent ingestion quantities.

Ingestion rates assumed for this analysis based on LaGoy 1987) are

100 mg/d for the maximum adult individual, 500 mg/d for the maximum child,

25 mg/d for the average adult, and 100 mg/d for the average child. The

quantity of contaminants ingested is dependent on the concentration of the

contaminants in the surface dust or dirt layer, or in other words, on the

32

assumed mixing layer. Assumptions concerning the mixing layers, noted

above, are 160 g/m 2 for a dust layer on pavement and 1600 g/m 2 ( mm

thick) for other surfaces.

Since the source term is in units of Ci per unit area, the ingestion

rate must be converted to an area basis. Ingestion rates based on area

then depend on the mass of dust layer calculated as follows:

2mg/d ingested X 9 x 10,000 cm cm2/d ingested

g/m2dirt 1000 mg m2

Ingestion rates based on contamination from a given surface area have been

used to assess ingestion of removable radioactive contamination residing

on surfaces. Table C.2 presents the area-equivalent contamination

ingested for use with the contaminant source term. The ingestion rates

used for this analysis are equal to the contamination from about 02

to 31 cm 2 /d.

These ingestion rates are reasonably comparable to rates calculated

by Healy 1971) to estimate ingestion doses. Healy assumed that I cm 2

of surface contamination could be taken into the mouth per hour; thus,2

ingestion rates for workers of 8.0 cm /d and for the public of

24 cm 2/d were assumed. The higher ingestion rate for the public was

presumed to allow for higher intake by children.

Table C.2 Ingestion Rate of Contaminated Soil for Area Based Source Term

Adult ChildSurface Mixing Maximum Average Maximum Average

100 mg/d 25 mg/d 500 mg/d 100 mg/d

Pavement 160 g/m2 6.3 cm2 1.6 cm2 31 m2 6.3 2Ground 1600 g/m2 0.63 m2 016 cm2 3.1 m2 0.63 m2

The intake rates given in Table C.2 are used to estimate dose from

ingestion of contaminants in the SST2 source term in the text of this

paper.

33

REFERENCES

DUGGAN, M. J. and S. Williams. 1977. "Lead-In-Dust in city St .reets.11The Science of the Total Environment. pp. 91-97. Elsevier ScientificPublishing Company, Amsterdam.

GALLACHER, J. E. J., P. C. Elwood, K. M. Phillips, B. E. Davies, andD. T. Jones. 1984. "Relation Between Pica and Blood Lead in Areas ofDiffering Lead Exposure." Archives of Disease in Childhood 59:40-44.

HAWLEY, J. K. 1985. "Assessment of Health Risk from Exposure toContaminated Soil." Risk Analysis 54):289-302.

HEALY, J. W. 1971. Surface Contamination: Decision Levels.LA-4558-MS, Los Alamos Scientific Laboratory, Los Alamos, New Mexico.

LAGOY, P. K. 1987. "Estimated Soil Ingestion Rates for Use in RiskAssessment." Risk Analysis 73):355-359.

LEPOW, M. L., L. Bruckman, M. Gillette, S. Markowitx, R. Robino, and J.Kapish.1975. "Investigations into Sources of Lead in the Environmentof Urban Children." Environmental Research 10:415-426.

ROELS, H. et al. 1980. "Exposure to Lead by the Oral and PulmonaryRoutes of Children Living in the Vicinity of a Primary Lead Smelter."Environmental Research 22:81-84.

SCHAUM, J. 1984. Risk Analysis of TCDD Contaminated Soil. Prepared forEPA Office of Solid Waste and Emergency Response by EPA Office ofHealth and Environmental Assessment, Washington, D.C.

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